It is known to produce viscose by treating sulfite pulp, produced from softwood, in the form of sheets or as a slurry with aqueous sodium hydroxide solution, mechanically pressing the resulting alkali cellulose to remove excess hydroxide solution, mechanically disintegrating the alkali cellulose and aging the disintegrated cellulose, during which the average degree of polymerization of the cellulose is degraded to a desired value--for example an average degree of polymerization of 300. The alkali cellulose with degraded molecular structure is then sulfided by reacting it with carbon disulfide. The cellulose xanthate formed during the chemical sulfiding reaction is then dissolved in sodium hydroxide solution, and the solution is subjected, if necessary, to another standing period for further aging, during which time the solution can be filtered and degassed in a known manner.
The amount of carbon disulfide required to sulfide the alkali cellulose with carbon disulfide to produce cellulose xanthate depends on the qualitative and quantitative structure of the initial cellulose which varies according to its source. The amount of carbon disulfide required also varies with the molecular weight distribution of the alkali cellulose to be sulfided and with the desired properties of the viscose produced in the process, depending on the intended use of the product. If the viscose is to be used in the manufacture of films, a smaller feed quantity of carbon disulfide is required for sulfiding alkali cellulose than is needed in the manufacture of viscose destined for the production of staple rayon. For high-strength fibers, the amount of carbon disulfide required for sulfiding may be twice as high as the amount needed in the manufacture of viscose to be used for producing staple rayon.
The aqueous NaOH solution containing dissolved cellulose xanthate generally contains from 4 to 10% by weight cellulose and from 4 to 7% by weight sodium hydroxide, relative to the total weight of the solution.
In sulfiding sodium cellulose with carbon disulfide, up to about 65 to 70% of the quantity of carbon disulfide supplied for this purpose is consumed in the desired reaction for the formation of cellulose xanthate, while about 25 to 30% of the total amount of carbon disulfide supplied is consumed in the formation of undesired by-products and about 3 to 5% by weight of the total quantity of carbon disulfide supplied during sulfiding remains chemically unconverted.
In the sulfiding reaction of alkali cellulose with carbon disulfide, the formation of by-products with a consumption of carbon disulfide is undesirable not only because of the uneconomical consumption of carbon disulfide and alkali metal hydroxide solution, but also because, during the further processing of the viscose solution, these by-products decompose and form noxious gases which interfere considerably with the course of production. For example, the troublesome hydrogen sulfide liberated during the process of spinning a viscose solution, is formed as a result of chemical decomposition of sulfur-containing by-products in the viscose solution.
For this reason, attempts have been made to control the visose-manufacturing process in such a way that the total quantity of carbon disulfide supplied for sulfiding a given quantity of alkali cellulose is utilized in an optimum manner to produce cellulose xanthate, and the formation of undesired by-products is largely suppressed.
Thus, a viscose-manufacturing process is known which serves this aim and wherein the alkali cellulose conventionally prepared in the first process step is, after mechanically pressing off the excess alkali metal hydroxide solution, shredding the alkali cellulose and ripening the shredded material, subjected to at least one further alkalization in which the NaOH concentration of the aqueous hydroxide solution, reacted with the alkali cellulose, is less than 15% by weight.
As a result of the second alkalization of the alkali cellulose which has been pressed off and then pre-aged, the amount of carbon disulfide required to sulfide the cellulose in order to prepare a viscose solution of defined properties is markedly lower than the amount needed to prepare corresponding viscose solutions when the process comprises only one alkalization step with a sodium hydroxide solution of relatively high concentration.
When the quantity of carbon disulfide supplied for sulfiding alkali cellulose is utilized to the optimum, however, this known viscose-manufacturing process comprises the following sequence of the process steps:
(a) first alkalization of the cellulose PA1 (b) mechanically pressing-off excess hydroxide solution from the alkali cellulose PA1 (c) shredding the alkali cellulose PA1 (d) ripening the alkali cellulose PA1 (e) alkalizing the ripened product of step (d) PA1 (f) mechanically pressing-off excess hydroxide solution from the alkali cellulose PA1 (g) shredding the alkali cellulose PA1 (h) sulfiding the alkali cellulose PA1 (i) dissolving the cellulose xanthate and PA1 (j) further aging the viscose
It will be appreciated that such a process has the disadvantage of being very involved and uneconomical.